Metal particles with an average particle size of less than 1 μm, in particular 1 to 100 nm, are called “metal nanoparticles”. Metal nanoparticles are confirmed to have high bondability caused by the fine particle size and to result in bonds between particles at a far lower temperature than the melting point of the metal forming the nanometal particles. Further, the structural strength of the obtained bonds is expected to be maintained until close to the melting point of the metal. As the metal forming the metal nanoparticles, Ag is typical. In addition, Au, Cu, Ni, etc. may be mentioned (for example, PLT 1).
Metal nanoparticles are generally used as organic-metal composite nanoparticles comprised of metal nanoparticles covered by organic shells. At room temperature, the organic shells can prevent self aggregation of nanoparticles and maintain their independent dispersed form. If heating the nanoparticles supplied to the surfaces of joined members, which are the members to be joined, the organic shells are broken down and removed, the low temperature sintering function of the surface active metal nanoparticles is exhibited, and the particles are sintered and simultaneously bonded with the surfaces of the joined members.
In the field of power semiconductors, power semiconductor modules comprised of semiconductor chips etc. fixed to boards are being applied in various fields. Here, as the art which is used for connecting the semiconductor chips and boards, in the past soldering has been used.
On the other hand, along with the recent technical advances in the field of power semiconductors, realization of energy saving power devices is expected by enabling use at a higher temperature. In this regard, since soldering has been used for joining power semiconductor modules, there has been the problem that the joining structure could not be secured at a high temperature. Therefore, art is being developed for utilizing the high bondability of metal nanoparticles for use as the joining material for semiconductor chips etc. (PLTs 2 and 3).
When joining two members, if one or both of the surfaces of the joined members are Al, joining them becomes difficult. When using Ag nanoparticles for joining metal, if the metal is Al, the joining strength becomes extremely low. The oxides at the Al surface are stable, so it is believed that at the time of joining, the oxide film could not be removed. In PLT 1 as well, it is described that Al etc. are hard to join with Ag nanoparticles.
In joining an Al electrode and an Au bonding wire of a semiconductor device, the ultrasonic welding method has been used. By pressing an Au bonding wire against an Al electrode while applying ultrasonic waves, the oxide film on the Al electrode surface is broken and a metal bond between the Al electrode and Au bonding wire is made possible. In this regard, if placing the joined semiconductor device in a high temperature environment, voids are formed in the intermetallic compound phase of the joint, so the combination of Al and Au, while initially giving a high joining strength, is not suitable for use in a high temperature environment. At the time of joining Cu to Al as well, again voids are formed and deterioration results.